Glass and ceramic articles may be decorated or imaged with printed decals. Such decals are typically comprised of flexible substrates and thin transferable coatings or film. The desired image or decoration is first printed upon the transferable coating or film side of the decal. The image or decoration is then transferred to the ceramic or glass article along with the transferable coating or film it is printed upon. The ceramic or glass article is then fired to permanently affix the image or decoration to the glass or ceramic article.
Image transfer from the decal to the glass or ceramic article may be accomplished by first removing the flexible substrate from the imaged transfer layer or film and then placing it on the article in the desired location. Such a process may be facilitated by using a water slide decal which contains a thin water soluble layer between the flexible substrate and the transfer layer. By soaking such a decal in water, the imaged transfer layer is easily separated from the flexible substrate and placed on the article to be decorated.
Decals incorporating a heat-meltable layer may be used to thermally transfer the image from the decal to the article. In this thermal transfer process the imaged transfer layer is easily separated from the flexible substrate at elevated temperatures and transferred either directly or indirectly to the article to be decorated or imaged. During the heat transfer step, the image and transfer layer are never unsupported as is the case in the water slide process.
The applicants have discovered that pressure sensitive adhesives may be used to facilitate the transfer of the imaged transfer layer from the decal to the article to be decorated or imaged. This new process eliminates the need for a heat-meltable layer in the decal and enables the process to be conducted under ambient temperature conditions. Like the heat transfer process, the imaged transfer layer is never unsupported in the pressure sensitive adhesive transfer process. However, in this adhesive transfer process, direct transfer of the imaged transfer layer to the article is preferred.
Processes for preparing “decals” are well known. Thus, e.g., in U.S. Pat. No. 5,132,165 of Louis A. Blanco, a wet printing technique was described comprising the step of offset printing a first flux layer onto a backing sheet, forming a wet ink formulation free of glass and including a liquid printing vehicle and oxide coloring agent, wet printing the wet ink formulation onto the first flux layer to form a design layer, and depositing a second flux layer onto the design layer.
The process described by this Blanco patent is not readily adaptable to processes involving digital imaging, for the wet inks of this patent are generally too viscous for ink jet printing and not suitably thermoplastic for thermal transfer or electrophotographic printing.
Digital printing methodologies offer a more convenient and lower cost method of mass customization of ceramic articles than do conventional analog printing methodologies, but they cannot be effectively utilized by the process of the Blanco patent.
The Blanco patent issued in July of 1992. In September of 1997, U.S. Pat. No. 5,665,472 issued to Konsuke Tanaka. This patent described a dry printing process that overcame some of the disadvantages of the Blanco process. The ink formulations described in the Tanaka patent are dry and are suitable to processes involving digital imaging.
However, although the Tanaka process is an improvement over the Blanco process, it still suffers from several major disadvantages, which are described below.
The Tanaka patent discloses a thermal transfer sheet which allegedly can “ . . . cope with color printing . . . ” According to Tanaka, “ . . . thermal transfer sheets for multi-color printing also fall within the scope of the invention” (see Column 4, lines 64-67). However, applicants have discovered that, when the Tanaka process is used to prepare digitally printed backing sheets for multi-coloring printing on ceramic substrates, unacceptable results are obtained.
The Tanaka process requires the presence of two “essential components” in a specified glass frit (see lines 4-12 of Column 4). According to claim 1 of U.S. Pat. No. 5,665,472, the specified glass frit consists essentially of 75 to 85 weight percent of Bi2O3 and 12 to 18 weight percent of B2O3, which are taught to be the “essential components” referred to by Tanaka. In the system of this patent, the glass frit and colorant particles are dispersed in the same ink. It is taught that, in order to obtain good dispersibility in this ink formulation, the average particle size of the dispersed particles should be from about 0.1 to about 10 microns (see Column 4 of the patent, at lines 13-17).
In the example presented in the Tanaka patent (at Column 7 thereof), a temperature of 450 degrees Celsius was used to fire images printed directly from thermal transfer sheets made in accordance with the Tanaka process to a label comprised of inorganic fiber cloth coated with some unspecified ceramic material.
When one attempts to use the process of the Tanaka patent to transfer images from a backing sheet to solid ceramic substrates (such as glass, porcelain, ceramic whitewares, etc.), one must use a temperature in excess of 550 degrees Celsius to effectively transfer an image which is durable. However, when such a transfer temperature is used with the Tanaka process, a poor image comprised with a multiplicity of surface imperfections (such as bubbles, cracks, voids, etc.) is formed. Furthermore, when the Tanaka process is used to attempt to transfer color images, a poor image with low color density and poor durability is formed. The Tanaka process, although it may be useful for printing on flexible ceramic substrates such as glass cloth, is not useful for printing images on most solid ceramic substrates.
It is an object of this invention to provide a ceramic decal assembly which, after being fired, produces durable images on a ceramic substrate, wherein the optical quality of the fired images is substantially as good as that of the unfired images.